1. Technical Field
The present disclosure relates to a method for the uniform distribution of photonic energy to a culture in a fluid medium that overcomes the problems of the turbidity of a dense culture and of bio-filming.
2. Description of the Related Art
The rapid and high density growth of photosynthetic microorganisms is critical in many industrial applications. These industrial applications include, without limitation, providing biomass used to extract biofuel, providing biomass used to produce methane by anaerobically digesting the biomass, producing food or specialized nutriceuticals, such as EPA and DHA for animals and humans, producing food and a growth environment for aquaculture, and consuming and sequestering industrial waste products such as CO2. Providing faster growth and producing high density cultures is critical to achieving the operational scale necessary for current environmental and industrial needs. Ideally, improving the speed of growth and increasing the density of a culture will require less production space and consequently will lower the cost of associated facilities.
Effective control of light. To maximize the growth of photosynthetic organisms, light must be available at the right intensity, the right frequency, and without excessive heat. Excessive light intensity can limit growth by inducing photo-respiration or bleaching the pigments needed for efficient cell growth. In addition, light intensity or light frequency in excess of the culture requirements may result in heat build-up that can limit culture growth. These problems are readily apparent in production systems that rely solely on direct solar light as a driver of photosynthesis, such as in ponds and raceways. Solar light is subject to extreme diurnal and seasonal variability. In addition, cultures relying on direct solar light are subject to periodic heating from light intensities and spectra not immediately useable by the culture.
The ability to effectively control light for 24 hours per day encourages faster growth of biomass and secondary metabolites as well as continuous consumption of CO2. This results in a more efficient use of facilities, enabling a smaller footprint for a given level of production. However, the use of artificial light has a cost that must be minimized for successful industrial application.
Obstacles to light delivery. Two major obstacles that reduce the uniform delivery of light to a culture are turbidity and biofilming. Turbidity occurs as a culture approaches a density where some of the organisms shade others from the light. Ensuring delivery of the appropriate amount of light to each organism becomes increasingly difficult as the density of the organisms in a culture increases. Turbidity within a culture results in some organisms receiving less light than they can use and non-productive absorption of light by other organisms. The absorption of excess light wastes energy and contributes to heat build-up.
Biofilming is an extremely widespread problem that occurs when a microorganism adheres to a surface. Most microorganisms, including species in all three domains, i.e., bacteria, eukaryotes, and archaea, perform processes that result in adhesion to surfaces and to other microorganisms. In industrial applications, biofilms often clog or corrode pipes and surfaces. In photo-bioreactors a biofilm can form over a light-delivery surface, thereby reducing the intensity and changing the spectra of the light transmitted.
Current techniques that address turbidity utilize tubular bioreactors that illuminate the culture as it flows through pipes of sufficiently narrow diameter. However, as the diameter of the pipes decrease, the friction and head pressure increase, requiring more powerful pumps to move the culture through the pipes. In addition, the large surface area of the pipe relative to the amount of culture exposed to the light increases the occurrence of biofilming. Some existing tubular systems periodically interrupt production and send pigs through the pipes to remove the biofilm from the interior surface of the pipe. Other systems include increasing the speed of flow and incorporating beads in the flow to increase turbulence and thus reduce biofilming. Increasing the speed of circulation requires longer pipes to insure that the culture is given sufficient light. All of these solutions increase the power required to maintain flow and thereby inflate the cost of production.
Another technique used to address turbidity in cultures includes exposing the culture to the light source as a shallow and wide flow. Generally in such systems biofilming is not a problem because no surface exists between the culture and the light source. However, the broadcasting of light over such a wide target prevents fine control of the intensity delivered and requires significant amounts of power to produce the required amount of light. The power cost is not a factor when direct solar radiation is used, but the process is subject to the variability entailed by solar radiation. When artificial light is used, the excess energy expenditure for the wide broadcast of light greatly increases the cost of production.
A third technique is described in Eckelberry et al., U.S. Patent Application No. 2009/0029445. The technique is to use a closed bioreactor, relying on an efficient source of artificial light, such as LEDs. The effects of turbidity are reduced by providing paddles that produce circulation. However, such a system is still subject to biofilming of the light sources.
The present disclosure addresses the costs added from use of artificial light by reducing the amount of light needed to maximize growth and preventing the obstruction of light sources by biofilming. Uniform and efficient distribution of light in the culture and the optimization of the spectra and intensity of light based on the particular organism being grown, the purpose of growing them, and their stage of growth helps to maximize growth of photosynthetic organisms.